A liner hanger for use in hanging liners in a well bore typically involves an inclined and annular slip expander surface with an inclination at an angle of 5 to 7 degrees relative to a lengthwise axis and complimentarily inclined and circumferentially spaced wall engaging slips slidably mounted on the expander. In some instances the expander is only circumferentially spaced inclined surfaces rather than a continuous annular surface. Use of circumferentially spaced expander surfaces enhances the fluid bypass about the surfaces.
The slips in a liner hanger are initially releasably retained in a longitudinal, spaced apart position from the expander surfaces until the liner hanger is in a location in a well bore or casing where the slips are intended to be set. The liner hanger is typically attached to the upper end of a string of pipe joints called a "liner" and the liner hanger, in turn, is releasably coupled to a setting tool. The setting tool, in turn, is connected to a length of drill pipe which is made up, section by section, to the earth's surface. The liner has an O.D. (outer diameter) as large as possible to pass through the I.D. (inner diameter) of any pipe already cemented or located in a borehole or any borehole located below a pipe in the well bore. This means that the annular space between the O.D. of a liner and I.D. of a borehole or pipe already in a well bore is kept as small as possible. At the lower end of the liner is float equipment which provides cementing back pressure valves.
Because of the annulus between the liner and a borehole or pipe has a small clearance, any obstruction in the annulus clearance or reduction of the effective open area in the annulus causes resistance to downward movement of the liner and liner hanger while it is lowered into the well bore through the mud in the well bore. This slows down the operation of moving the liner and liner hanger to the desired location. One of the critical areas of blockage in the annulus between a liner hanger and a well casing or well pipe is the slip and expander construction which projects outwardly of the overall diameter dimension of the liner hanger when it is set and restricts the annular fluid flow during the cementing operation.
As noted above, the slips are initially in a retracted condition relative to an expander surface. At the location in a well bore where the liner hanger is hung, the slip members are released and moved relative to the expander surface and by virtue of engagement the inclined expander surface with the inclined surface of a slip, the slips will move outwardly into engagement with the wall of a well bore and "hang" or support the weight of the liner. The weight of the liner is transmitted through the slip to the contact surface with the expander surface and develops a radial force on the slips and the expander surface. The radial force on the expander is equal and opposite to the radial force of the slips to the casing and is inversely proportional to the slip angle. Heavy liner loads can actually radially punch or distort the casing or can collapse an expander. One of the problems, where it is desired to rotate a mandrel on which an expander is rotatably mounted, is that radial contraction of the expander "seizes" or frictionally grips the mandrel so that rotation of the mandrel relative to the expander is not possible.
The expander and slip angle of inclination is maximized at usually from 5 to 7 degrees to minimize the radial forces on the expander and on the casing and also to keep the contact surface between a slipp and expander surface as large as possible so as to minimize the contact pressure (PSI). As noted, the angle of inclination affects the effective surface area between an expander and slip in load supporting contact. Effective fluid bypass for fluid around the slip and expander construction is affected by the circumferential size of the slips projecting outwardly of the expanders. Thus, for supporting purposes, the maximum slip area possible is achieved by a full-circle (360.degree.) of slips. But this combination results in no annular area for fluid bypass. It is therefore apparent that current slips designs are limited in their ability to minimize contact pressure due to the interrelationship of slip angle and the slip area and the direct effect that an increase in the bypass area will proportionately decrease the slip area.
Heretofore, as disclosed in U.S. Pat. No. 4,712,615, issued Dec. 15, 1987 and assigned to the assignee of the present invention, a slip and expander arrangement is illustrated wherein an elongated vertical "window" in a liner mandrel and an interfitted slip in a window are provided with inclined tongue and groove interconnections along the length of a slip member and a liner "window". The inclinations can be much greater than conventional slip and expanders and can be 15 degrees relative to a longitudinal axis. This structure results in radial forces two to three times lower than 5.degree. and 7.degree. slips. Additionally the weight load of a liner can be distributed along an unlimited vertical length of the outer surface of a slip since the slip length is not determined by the slip angle. This provides greater contact surface for reducing radial load on a casing without the need to decrease annular fluid bypass area.
As a result, slip contact pressures can now be held to a minimum with no negative effect on bypass.